Narcotics, 255. Natural gas, 152. Needle, magnetic, 328. Negative, electrode, 308. photographic, 130. Neutral substance, 222. and litmus, 222. Neutralization, 222. Niagara Falls, 176. Nitrogen, 66. and bacteria, 263. and plant life, 261. in atmosphere, 261. in fertilizer, 262-265. in food, 66. preparation of, 261. properties of, 261. Noise in music, 280. Non-conductors, of electricity, 321-322. of heat, 25. Nutcracker, as a lever, 162. Oboe, 297. Octave, 284. Odors, 101. Ohm, unit of resistance, 345. Oil, gasoline, 149, 150. kerosene, 149, 150. lubricating, 174. olive, 16. Orchestra grouping, 299. Ore, 72. Organ pipes, 297. Overtones, 290-293. Oxalic acid, 247, 248. Oxidation, 45-59. and decay, 49. heat the result of, 49-52. in human body, 49, 53. of iron, 48. Oxygen, 66. and bleaching, 239. and combustion, 45. and food, 66. and plants, 55. and the human body, 50. and water, 79, 80. in the atmosphere, 45. preparation of, 46. Paint, as wood and metal preservatives, 253, 254. removal of stains, 243. Paper making, 219. Paraffin, 150, 321. Pasteurized milk, 250. Patent medicines, 257-260. Peas, sources of nitrogen, 263. Pelton wheel, 177. Percussion instruments, 299. Period of a body, 273. Peroxide of hydrogen, 53, 252 Petrolatum, 150. Petroleum, 149, 150. Phonograph, 303-305. Phosphorus, in fertilizer, 263, 264. in making nitrogen, 261. in matches, 47, 48. poisoning by, 47. Photography, 127-131. Photometer, 107. Pianos, 284-292. Pin wheel, 181. Pitch of sound, 280, 281. cause of, 282. in wind instruments, 296-299. Plane, inclined, 162-166. Plants, and atmosphere, 55. and light, 131-132. and nitrogen, 261. Plate developing, photographic, 128. Pneumatic dispatch tube, 205. Poles, magnetic, 330-332. of cell, 308. Positive electrode, 308. Potash, in fertilizer, 263-265. Potassium chlorate and oxygen, 46. permanganate, 100. tartrate and Rochelle salt, 227. Power, candle, 105-107. electric, 351. horse, 173, 351. sources of, 174, 175, 185. transmission by belts, 171. water, 176-180. Precipitation, 72, 73. Preservatives, food, 252. wood and metal, 253-254. Pressure, atmospheric, 82-86. calculation of atmospheric, 83, 84. calculation of gas, 92, 93. calculation of water, 94. gauge, 92-94. of illuminating gas, 93. relation of pressure of gas to volume, 95, 96. water pressure, 208-211, 214-216. within the body, 86. Primary colors, 135. Print, photographic, 131.